Different methods and devices are known for counting microorganism colonies in, for example petri dishes or in thin film culture plate devices such as PETRIFILM.TM., manufactured by 3M of St. Paul, Minn. In the latter case, such devices have a very thin layer of growth medium, making all colonies visible with surface illumination. Manual counting of colonies on such media by trained laboratory personnel is well-known; typically a film will be inoculated and marked as to the source of the inoculant, stacked together with similar samples, and placed into an incubator. A manual inspection and counting is performed after a period of 12 to 24 hours. This method has known disadvantages, particularly the costs associated with the use of skilled technicians to perform the time-consuming task of manual counting, as well as the limited accuracy of the counts achieved.
Also desirable in the matter of microbiological counting is the early detection of colonies, particularly when food products are being tested. If the samples indicate excessive contamination, the product must often be discarded. Reliable early detection of excessive contamination in the range of 6 to 12 hours after inoculation would be welcomed by manufacturers because it would allow them to identify contaminated products early in processing, thereby avoiding additional expenses incurred in processing product that will be discarded and possibly contaminating additional product by running it through contaminated processing equipment.
International Publication No. WO 94/26870, which is hereby incorporated by reference, discusses improvements in colony counting in e.g. disposable microorganism culturing media having a substantially planar substrate. These include scanning and imaging the inoculated surface more than once and processing the images to produce a scaled time lapse image. Processing this scaled time lapse image allows the identification of hit pixels which can be clustered to identify the appearance of colonies. Thus early indication of colony growth can be obtained.
U.S. Pat. No. 5,403,722, which is also incorporated by reference, discloses a method and an apparatus for automated counting. However, a limitation on the apparatus disclosed therein is the physical properties of the disposable microorganism culturing media. Optimally, the art could wish for a more robust handling of the planar substrates, which have to be manipulated as a loose pile.
International Publication No. WO 95/16768, which is also incorporated by reference, discloses a specialized tool for handling disposable media under automatic control. The apparatus has an imaging means for detecting colonies on the disposable media, which media are contained within individual holders. Cooperating with the imaging means is a holder positioning means for storing and queuing one or more of the holders. The holder positioning means is adapted for moving the holders sequentially into a predetermined position relative to the imaging means so that images can be obtained. While this system provides a very accurate early detection, it is more complicated and expensive than is required in all industries where microbiological assays are performed.